1. Field of the Invention
The present invention relates to a method and an apparatus for electrical testing of microwired structures with the assistance of particle probes in which a first circuit node is electrically charged with at least one first particle beam and the second circuit node is investigated with at least one second particle beam to determine whether the second node is electrically connected to the first node.
2. Description of the Prior Art
Needle adapters which encounter their technological limit given a test grid whose grid dimension amounts to less than 200 .mu.m are presently employed for the electrical testing of printed circuitboards. Alternative testing methods are therefore being sought for testing miniaturized printed circuit-boards and for testing microwired structures.
The German published application No. 22 15 179, fully incorporated herein by this reference, discloses a method for electrical testing of printed circuitboards with the assistance of two electron probes. For example, for checking a resistance value, a first end of the resistor to be measured is charged to a first potential with the assistance of a first electron probe. The secondary electron emission coefficient at the point of incidence of the first electron probe must thereby be less than 1. The other end of the resistor to be measured is driven by a second electron probe. The secondary electron emission coefficient at the point of incidence of the second electron probe must be greater than 1. In the static condition, both the point of incidence of the first electron beam as well as the point of incidence of the second electron beam respectively have a defined potential. In the static condition, the current having a very specific current intensity also flows between these points of incidence of the two electron probes. Given this known method, a secondary electron current which likewise has a very specific current intensity is emitted at the point of incidence of the second electron probe in the static condition. Given this known method, the value resistance to be identified is determined in that the potentials of two points of incidence of the two electron probes and the current intensity of the second electron current emitted from the point of incidence of the second electron probe are identified.
A method that is the same as the method disclosed in the above-mentioned published German application is disclosed in U.S. Pat. No. 4,415,851, fully incorporated herein by this reference. A method that is similar to that of the German published application is disclosed in U.S. Pat. No. 4,417,203, also fully incorporated herein by this reference.
Given the known electron beam measuring methods for checking microwired structures, interconnect networks are electrically charged with at least one electron probe. When there is a conductive connection between circuit nodes that are electrically charged or, respectively, that have been electrically charged, and other circuit nodes, then electrical charge is also transported to the other points via this conductive connection. As a consequence, the potentials of these other points are also changed. The changed potentials of the other points can be measured in the known electron beam measuring methods via the secondary electrons that are emitted from these other points. The energy of the secondary electrons emitted from a test access point depends on the potential of the test access point (circuit node). Conductive connections can be discriminated from interrupted connections with such a method. A measure for the potential at a measuring point is the size of the secondary electron signal read at the measuring point. Measuring the potential at a measuring point via the secondary electron signal belonging to the measuring point is disclosed, for example, in U.S. Pat. No. 4,277,679, fully incorporated herein by this reference. The secondary electron signal depends not only on the potential of the measuring point, but also, in an undesired manner, on close fields in the environment of the measuring point, on the contamination in the environment of the measuring point and on the respective electron orbits which lead to the detector and which generally proceed differently for different energies of the secondary electron beams.
When different energies of the primary electrons are required for writing a potential at a circuit node and for reading the potential at a different circuit node, then the switching of the beam generator to different primary electrons is only possible with great technical expense. Finally, the interconnected networks have different capacitances and therefore require different times until specific circuit nodes are respectively charged to a specific voltage. When measuring an individual value of resistance given the aforementioned measuring methods, therefore, either a relatively long time must be provided for the charging event so that one can be certain that the measuring arrangement is in the static condition or an involved apparatus must be provided so that the charging event can also be tracked in a measuring fashion with one of the above measuring methods.